Redundant power supplies are used in networking equipment to increase the overall reliability of the associated network. This improvement in reliability is available through several mechanisms. The first mechanism is the automatic isolation of failed power supply elements and load elements from the power source. The power source comprises a plurality of individual power supplies, each capable of isolation from the other supplies. The load comprises a plurality of individual loads, each capable of isolation from the other loads. The combination of supply isolation, load isolation, and isolation control means provides a system with composite reliability improved over prior art apparatus and methods.
The prior art for current sharing power supplies is extensive. This prior art in the area of redundant current sharing power supplies falls into three general areas: droop sharing power supplies, 3 wire control power supplies, and local sensing power supplies. The first area pertains to droop sharing power supplies, as in U.S. Pat. No. 4,924,170 (Henze) wherein the output impedance of the supply is used to share load current. Disclosed in this patent is local feedback of output current as a term which has the overall effect of increasing the output impedance of the supply for DC, while maintaining a low output impedance at higher frequencies. An example of droop sharing with the use of non-linear feedback for improved droop range is Chesavage U.S. Pat. No. 5,834,925. The second area is 3 wire control power supplies, wherein the output of a high gain error amplifier is fed commonly to low gain output stages to produce an common output, which requires sharing of internal signals in addition to the usual combined outputs. One example of this is U.S. Pat. No. 4,734,844 (Rhoads et al) which describes a 3 wire regulation system wherein a master supply generates a control output, and a plurality of slave units act on this common control signal. This system is has the weakness that if one of the supplies contaminates the common control signal with erroneous input, the entire system will replicate and produce an erroneous output. Rhoads does not address redundancy in the sense of immunity to component failure, but shows additional interconnections between supplies for them to work properly. U.S. Pat. No. 5,521,809 (Ashley et al) discloses a current sharing circuit based on the power supplies exchanging information with each other relating to the level of current sharing through a separate bus wire, identified in the patent as a sharebus. Each power supply has a local estimate of current being delivered, which is compared with a fraction of the total current, and a local feedback term is provided to each supply to achieve current sharing. This method affords a high degree of accuracy in current sharing, but does not address either on-line redundancy or transient behavior.
A related method combining aspects of the first and third class of sharing is shown in U.S. Pat. No. 4,618,779 (Wiscombe) which describes a scheme for regulating a plurality of power supplies by modulating the value of the sense resistor in the feedback loop via an external controller which modulates this value based on sensing current delivered by each supply to the load.
The third area is local sensing power supplies, in which a locally sensed version of the output signal is compared with the total output current, and the local error signal represents a combination of output error signal and current sharing error. U.S. Pat. No. 4,035,715 (Wyman et al) describes a current sharing system wherein the total system output current is made available to each supply so as to ensure that each supply does not furnish more than its proportion of total load current. U.S. Pat. No. 5,552,643 (Morgan et al) describes a method of current summing wherein multiple switch mode power supplies deliver current to a common inductor. This addresses a method of current summing, but does not afford redundant operation. U.S. Pat. No. 4,257,090 (Kroger et al) describes a current sharing system wherein feedback is provided to each power supply based on the sum of the output voltage and a local measurement of inductor current, which ensures that each power supply is operating below the maximum current as constrained by a saturated output inductor. U.S. Pat. No. 5,477,132 (Canter et al) is similar to Ashley, and discloses means for measuring a total current, and delivering this measurement to the individual power supplies, which compare this total value to their individual contribution, and produce a local error term which is summed into the regulation loop along with the global (output) voltage regulation term. U.S. Pat. No. 4,866,295 (Leventis et al) describes another technique for current sharing based on measurement of output current from each supply being subtracted from a total measured output, similar to that described by Canter and Ashley. U.S. Pat. No. 4,766,364 (Biamonte et al) discloses a redundant power supply having a common output filter and distributed diode and inductor energy storage circuits. In this master/slave configuration, the master power supply computes an error signal that is distributed to the slave units. Each power supply further has decision circuitry to take that unit off-line if there appears to be a failure in that unit. A master error causes each slave supply to furnish its own local error signal and ignore the master signal.
The prior art for distributed power systems includes load isolation of U.S. Pat. No. 5,053,637 by Dillard, which shows distribution of power to a plurality of loads, each having overcurrent protection, however, no means of current sharing is shown.
Paralleling of power sources is disclosed in U.S. Pat. No. 5,774,736 by Wright et al where power sources operate in parallel, and additional independent sources are connected in parallel if the output voltage requirements are identical. The condition of shorted outputs in power supplies is not discussed.
Removal of power supplies in the event of failure is discussed in U.S. Pat. No. 5,122,726 by Elliott et al. In this patent, a power fault monitor in the power supply activates a fault signal which takes the power supply off line. The isolation element used is a diode, rather than an active switch element.
Switch based isolation elements are discussed in Darty et al in U.S. Pat. No. 5,752,047. A Solid State Power Controller (SSPC) is controlled by a microprocessor, which is examining the level of output current drawn by a load.